1. Field of the Invention
The present invention relates to an image forming system such as a copying machine, a facsimile, or a printer and more particularly to an image forming system wherein an image bearing member and an intermediate transfer member are kept in contact with each other, toner images formed on the image bearing member are primarily transferred onto the intermediate transfer member, and the toner images thus primarily transferred onto the intermediate transfer member are together transferred secondarily onto a transfer medium.
2. Description of the Prior Art
Heretofore, as this type of an image forming system, there is known an image forming system wherein toner images formed on a photoreceptor as an image bearing member are primarily transferred onto an intermediate transfer member by utilizing an electrostatic force and thereafter the toner images on the intermediate transfer member are secondarily transferred onto a transfer paper by utilizing an electrostatic force. The image forming system using such an intermediate transfer member is advantageous in that images can be formed on various kinds of transfer papers, including plain paper and cardboard.
Among image forming systems having an intermediate transfer member, there is known one which is constructed so as to perform image formation in a constantly contacted state of the intermediate transfer member with a photoreceptor. This image forming system does not require the provision of an engaging/disengaging mechanism for the engagement and disengagement of the intermediate transfer medium with respect to the photoreceptor. Thus, the image forming system in question is advantageous in that the components' cost can be reduced by an amount corresponding to the disengaging mechanism and hence space-saving can so much be attained in comparison with an image forming system which has a construction permitting engagement and disengagement of the intermediate transfer member with and from the photoreceptor.
Moreover, in the image forming system having an intermediate transfer member, toner images formed on a photoreceptor can be overlapped onto the intermediate transfer member, thus permitting the image forming system to be widely used as a color image forming system capable of forming a color image on a transfer paper. In the color image forming system there is used a developing apparatus having plural developing units capable of effecting development using different colors of developers, and latent images are developed by corresponding developing units respectively. In such an image forming system, when latent images formed successively on a single photoreceptor are developed by corresponding developing units respectively, there arises the necessity of switching from one to another developing unit in a successive manner. In connection with this developing unit switching operation, there are known a method wherein one and same developing position is used for the developing units and the developing units themselves are moved successively for development to the developing position and a method wherein different developing positions are used for the developing units respectively and the developing units themselves do not move. As examples of the former method are mentioned a revolver method and a slider method. On the other hand, as an example of the latter method there is known a method wherein switching is made from one to another developing unit by bringing only the developer in the developing unit to be used into contact with a photoreceptor with use of an engaging/disengaging mechanism provided in each developing unit.
In a developing apparatus adopting a revolver method or a slide method wherein developing units themselves are moved to a developing position, the developing units are generally arranged so that they can move to the developing position successively in accordance with a development order adopted in forming a full-color image in which image formation is performed using all the developing units. For example, in a revolver developing apparatus adopting a revolver method wherein development is performed in the order of black (“Bk” hereinafter), yellow (“Y”), cyan (“C”), and magenta (“M”) at the time of forming a full-color image, these four-color developing units are arranged side by side in their revolving direction in the order of Bk, Y, C, and M.
In the image forming system provided with such a revolver developing apparatus, the developing unit which is the first to make development must lie in the developing position before the start of development. For moving each developing unit most efficiently at the time of forming a full-color image, the revolver developing apparatus, before the start of the image forming process, is stopped at a home position where the Bk developing unit which is the first to perform development is located nearest to an upstream side in the revolving direction of the revolver developing apparatus. From this home position the developing unit which is used in this image forming process and which is the first to make development is moved to the developing position to effect development.
However, when the image forming process is started and the developing unit which is the first to make development has moved to the developing position, the developer in the developing unit adheres onto a photoreceptor already before the start of development, causing stain of the toner present on the photoreceptor. The first reason for the occurrence of such toner stain is presumed to be as follows. When the developer pressure increases between a developer carrier in a developing unit and a photoreceptor in relation to both a development gap and a developer scoop-up quantity (developer weight per unit area), there increases an impact force between the developer carrier and the photoreceptor with movement of the developing unit, so that it becomes easier for the toner to leave the developer carrier, and the spilt toner adheres onto the photoreceptor with van der Waals' force. The second reason is presumed to be as follows. The potential of a toner layer portion deposited on the developer carrier approaches a surface potential of the photoreceptor rather than the potential of the developer carrier surface, causing an effective bias of development to be changed, with consequent deposition of toner onto the photoreceptor. The toner stain thus generated is conveyed to a primary transfer section as it is adhered to the photoreceptor and adheres onto an intermediate transfer member. If the surface portion of the intermediate transfer member thus stained with the toner stain is an area (a “to-be-transferred area” hereinafter) onto which a toner image on the photoreceptor is to be primarily transferred in this image forming process, the toner stain overlaps a toner image to be subsequently transferred primarily onto the to-be-transferred area. The resulting ground stain causes deterioration of the image quality.
In the case where image formation is to be done using the above image forming system and using, for example, only the three colors of Y, C, and M without using Bk, it is necessary that the Y developing unit which is the first to make development be moved to the developing position after start of the image forming process. In this case, it is necessary that the Y developing unit be moved to the developing position while skipping over the Bk developing unit from the home position. During this movement, the developer in the Bk developing unit comes into contact with the photoreceptor surface at the developing position. Also at the instant of this contact the toner adheres onto the photoreceptor for the same reason as above and the resulting toner stain adheres onto the intermediate transfer member. If this stained portion is the to-be-transferred area, a ground stain results and causes an image quality deterioration like above.
In the case where image formation is to be performed using the above image forming system and using, for example, only the two colors of Bk and M, it is necessary that, after the completion of development by the Bk developing unit, the M developing unit to be used next for development be moved to the developing position. In this case, if a full-color image is to be formed, it is necessary that the M developing unit be moved to the developing position while skipping over the Y developing unit to be next used for development and further skipping over the C developing unit to be used for development next to the Y developing unit. At this time, if the to-be-transferred area in the surface movement direction of the intermediate transfer member is long, a not-to-be-transferred area of the intermediate transfer member becomes very narrow. Therefore, in relation to the developing unit moving time, even if the movement of the M developing unit is started just after the completion of Bk development, there sometimes occurs a case where the M developing unit cannot be moved to the developing position before a front end of the to-be-transferred area on the intermediate transfer member reaches the primary transfer section. In this case, M toner image cannot be primarily transferred onto the intermediate transfer member, so there arises the necessity of causing the intermediate transfer member to idle-rotate and thereby causing the to-be-transferred area to again reach the primary transfer section. During this idle-rotation, since the to-be-transferred area on the intermediate transfer member is long, C and M toner stains adhere to the to-be-transferred area on the intermediate transfer member, with consequent ground stain causing the deterioration of image quality.
Reference will be made below to a concrete example of image formation performed using an image forming system and using only the two colors of Bk and M. In the image forming apparatus used, the time required for switching from one to another developing unit in a revolver developing apparatus (the time required for 90° revolution) was 320 msec and a circumferential length of an intermediate transfer belt as an intermediate transfer member was 565.5 mm.
FIGS. 10(a) to 10(d) are timing charts showing a part of a conventional sequence control operation performed when an image is formed in a longitudinal direction of Japanese Industrial Standard A3-size paper (a direction in which the longitudinal direction of the paper is positioned in parallel with a surface movement direction on an intermediate transfer belt). FIG. 10(e) is a timing chart showing at what timing a to-be-transferred area on the intermediate transfer belt passes a primary transfer section. As shown in FIG. 10(e), Y toner stain and C toner stain adhere to a not-to-be-transferred area on the intermediate transfer belt, but M toner stain adheres within the to-be-transferred area. As a result, a lateral band-like ground stain attributable to M toner stain occurred in a portion 21 mm from the image front end on the paper.
When a 12×18 in. paper image larger than the Japanese Industrial Standard A3-size paper image was formed, C and M toner stains adhered to the to-be-transferred area on the intermediate transfer member and there occurred a lateral band-like ground stain attributable to the C toner stain at a portion 24 mm from the front end portion of the image on the paper and a lateral band-like ground stain attributable to the M toner stain at a portion 63 mm from the image front end. Further, when an image in the transverse direction of a Japanese Industrial Standard A4-size paper was formed by double-sheet image formation onto an intermediate transfer belt, there occurred a lateral band-like ground stain attributable to C toner stain at a portion 13 mm from the image front end on paper corresponding to the first sheet of image on the intermediate transfer belt and there occurred a lateral band-like ground stain attributable to M toner stain at a portion 50 mm from the image front end on the paper.